A large-scale storage is also called a storage subsystem, and in addition to making high-speed, large-capacity data storage possible, also comprises advanced data management functions. Multiple physical storage devices, such as hard disk drives, are mounted inside the storage. A logical volume, which is a logical storage area, is configured using the storage area inside these storage devices. A storage apparatus provides the logical volume to a host computer. The host computer reads and writes data with respect to the logical volume.
In recent years, thin provisioning technology, which enhances the capacity efficiency of the logical volume, has been proposed. Thin provisioning technology provides the host with a virtual logical volume (hereinafter, a virtual volume) instead of a conventional logical volume.
A conventional logical volume requires a physical storage area (a real storage area) of a size specified at volume creation. By contrast, in the case of a virtual volume, a real storage area for storing this data is fetched from a pool and allocated to the virtual volume at the point in time at which an actual data write has occurred. When thin provisioning technology is used, there is no need to allocate a real storage area to the virtual volume until the data write actually occurs, making it possible to conserve the real storage area.
However, depending on the state of the data write to a virtual volume, there could be a case in which the amount of data being written to the virtual volume exceeds the size of the real storage area capable of being allocated to the virtual volume. An error can occur when the real storage area to be allocated to the virtual volume is insufficient.
For this reason, when using thin provisioning technology, it is desirable that the size (the actual amount used) of the real storage area capable of being allocated to the virtual volume be precisely managed.
In a first prior art, in a case where there is a capacity shortage in a pool, the size of the pool is expanded by adding a usable volume to the pool (Patent Literature 1). In a second prior art, a capacity shortage in a migration-source pool is resolved by migrating a virtual volume that is using the pool to another pool (Patent Literature 2).